The effects of hypoxia on contraction and relaxation in chick embryo ventricular cells cultured as confluent, spontaneously contracting monolayers will be studied. Inotropic effects are detected by measurement of changes in the amplitude and velocity of individual cell wall motion. Similar changes in cell wall motion during diastole can be measured, and can be analyzed to detect the influence of hypoxia on parameters of relaxation. Negative inotropic effects of hypoxia will be correlated with changes in calcium contents and rapid Ca influx and efflux measured using 45Ca as a tracer. The effects of hypoxia on monovalent cation active transport, and on cellular contents of Na and K, will also be determined using radioactive isotopes of Na and K. These effects of hypoxia will be correlated with the effects of this intervention on cellular contents of ATP and creatine phosphate. Our overall goal is to determine the mechanism(s) of the negative inotropic effects of hypoxia and to investigate the cause of abnormal relaxation during this insult.